1. Field of the Invention
The present invention relates to a method for operating a magnetic resonance device in which a magnetic resonance signal is recorded for a span of time, and in which, in order to produce a magnetic resonance spectrum, the magnetic resonance signal in the time domain is subjected to a Fourier transformation.
2. Description of the Prior Art
Magnetic resonance spectroscopy has been used for more than four decades in basic research in physics, chemistry, and biochemistry, for example as an analytical technique or for the structural clarification of complex molecules. As is the case for magnetic resonance tomography, magnetic resonance spectroscopy is based on the principle of magnetic nuclear spin resonance. However, the primary goal of spectroscopy is not imaging, but rather an analysis of a material. Resonant frequencies of isotopes that have a magnetic moment, for example 1H, 13C, or 31P, are dependent on the chemical structure of molecules in which the above-named isotopes are bonded. A determination of the resonant frequencies therefore makes it possible to differentiate between different materials. The signal intensity at the various resonant frequencies provides information concerning a concentration of the corresponding molecules.
If a molecule is brought into a basic magnetic field of a magnetic resonance device, as takes place in spectroscopy, electrons of the molecule shield the basic magnetic field for atomic nuclei of the molecule. Due to this effect, the local magnetic field at the locus of an atomic nucleus changes by a few millionths of the external basic magnetic field. The accompanying variation of the resonant frequency of this atomic nucleus is called chemical displacement. In this way, molecules can be identified on the basis of their chemical displacement. Since, from the point of view of measurement technology, frequency differences can be acquired more easily and more precisely than can absolute frequencies, the chemical displacement is indicated in ppm relative to a reference signal, for example the operating frequency of the magnetic resonance device.
A resonance line of an atomic nucleus can be split into a number of lines if additional atomic nuclei having a magnetic moment are located in the environment of the atomic nucleus under observation. The cause for this is due to the phenomenon known as spin-spin coupling between the atomic nuclei. The magnetic flux density of the basic magnetic field that is experienced by an atomic nucleus depends not only on the electron shell surrounding this atomic nucleus, but also on the orientation of the magnetic fields of the neighboring atoms. Because the resolution capacity of the magnetic resonance device is often too low, the spin-spin coupling is thereby often not visible in the spectra.
Clinical magnetic resonance spectroscopy is understood to refer to magnetic resonance spectroscopy with the use of clinical magnetic resonance tomography devices. The methods used in localized magnetic resonance spectroscopy are distinguished from those used in magnetic resonance imaging essentially by virtue of the fact that in spectroscopy, in addition to the tomographic locus resolution, chemical displacement is resolved.
Currently, in clinical applications two types of localization methods for magnetic resonance spectroscopy are dominant. A first type includes individual volume techniques based on echo methods, in which a spectrum of a target volume selected beforehand on the basis of proton images is recorded. A second type includes spectroscopic imaging methods (Chemical Shift Imaging, CSI) that simultaneously enable the recording of spectra of a multiplicity of spatially contiguous target volumes.
The individual volume techniques standardly used today are based on acquisition of a stimulated echo or of a secondary spin echo. In both cases, a locus resolution takes place through successive selective excitations of three orthogonal layers. The target volume is defined by the slice volume of these three layers. Only the magnetization of the target volume experiences all three selective high-frequency pulses and thus contributes to the stimulated, or secondary spin echo. The spectra of the target volume is obtained through one-dimensional Fourier transformation of a time signal corresponding to the stimulated echo, or to the secondary spin echo.
Spectroscopic imaging methods are used both in clinical phosphorus spectroscopy and in proton spectroscopy. A 3D CSI pulse sequence includes for example the following steps: After a slice-layer-selective 90xc2x0 high-frequency pulse, for a defined period of time a combination of magnetic phase coding gradients of the three spatial directions is activated, and subsequently the magnetic resonance signal is read out in the absence of all gradients. This procedure is repeated as often as necessary with different combinations of phase coding gradients until the desired locus resolution has been achieved. A four-dimensional Fourier transformation of the magnetic resonance signals supplies the desired spatial distribution of the resonance lines. If the above-described non-selective high-frequency pulse is replaced by a slice-selective excitation, consisting of a frequency-selective high-frequency pulse and a corresponding magnetic gradient, one phase coding direction can be omitted, and given a 2D CSI pulse sequence of this sort the measurement time is reduced in relation to the 3D CSI pulse sequence.
In clinical proton spectroscopy, the intensive water signals are often suppressed using so-called water suppression methods. A method for water suppression is, for example, the CHESS technique, in which the nuclear spins of the water molecules are first selectively excited by narrowband 90xc2x0 high-frequency pulses, and their cross-magnetization is subsequently de-phased through the switching of magnetic field gradients. For an immediately subsequent spectroscopic imaging method, in the ideal case no detectable magnetization of the water molecules is therefore available. In methods using water suppression, however, metabolites are at least partially also saturated, so that, disadvantageously, these metabolites contribute only slightly, or not at all, to a magnetic resonance signal, and appear only weakly, or not at all, in the associated spectrum.
In the above-described individual volume techniques and spectroscopic imaging methods, magnetic resonance signals of a particular chronological length, for example a free induction decay or a second half of a spin echo, are subjected to a Fourier transformation, as a part of a very comprehensive procedurexe2x80x94from the point of view of magnetic resonance physicsxe2x80x94for the production of a spectrum. Among other things, this results in the resonance lines, in particular the water resonance lines, of a spectrum experiencing an undesired line broadening, so that metabolite resonance lines adjacent to the water resonance line are covered due to the broadening, and, in addition, a precise frequency determination in particular of metabolite resonance lines, and thereby the identification thereof as particular molecules, is made more difficult.
An object of the present invention is to provide a method of the type described above for the operation of a magnetic resonance device that reduces the cited disadvantages of the prior art.
The above object is achieved in accordance with the principles of the present invention in a method for operating a magnetic resonance device, wherein a magnetic resonance signal is recorded for a time span and wherein, in order to produce a magnetic resonance spectrum, the magnetic resonance signal in the time domain is subjected to a Fourier transformation, and wherein, before the Fourier transformation, the magnetic resonance signal is weighted with a bell-shaped window function.
Due to the fact that before the Fourier transformation the magnetic resonance signal is weighted with a bell-shaped window function, a line broadening of resonance lines is prevented. Frequencies, in particular of non-dominant resonance lines, can thereby be precisely determined, so that an unambiguous identification of the associated materials is enabled. A bell-shaped window function is a function that for all times outside a window width, it has the value zero, and inside the window width it has a bell-shaped curve having values different from zero.
In one embodiment, the bell-shaped window function is a symmetrical window function, for example a Hanning window function. The bell-shaped curve of the symmetrical window function is mirror-symmetric to an axis running through a midpoint of the window width. In additional embodiment, other bell-shaped window functions can be used, such as are for example described in the article by F. J. Harris, xe2x80x9cOn the Use of Windows for Harmonic Analysis with the Discrete Fourier Transform,xe2x80x9d Proceedings of the IEEE, vol. 66, no. 1, January 1978, pp. 51-83. Here reference is made to the fact that the rectangular window function does not present a bell-shaped window function.
In another embodiment, the magnetic resonance signal is recorded free from a suppression of a dominant resonance line. Due to the fact that in the inventive method no line broadening of the dominant resonance line takes place, none of the non-dominant resonance lines adjacent to the dominant resonance line are covered by a broadened dominant resonance line, so that the above-cited suppression can be omitted, which suppression is known to have the disadvantage that, besides the dominant resonance line, adjacent non-dominant resonance lines are also at least partially suppressed.
In a specific embodiment of the invention, the magnetic resonance signal recorded for the span of time is present in the form of a discrete data set, for example stored in a computer system of the magnetic resonance device. For the production of the spectrum, the Fourier transformation is thereby applied to the discrete data set as a discrete Fourier transformation, for example as a fast Fourier transformation (FFT). The discrete data set is thereby weighted, before the Fourier transformation, with a discrete bell-shaped window function, such discrete window functions being produced by discretely dividing comparable time-continuous window functions.
In a further embodiment, the recording of the magnetic resonance signal is started at the earliest possible point in the time sequence of a sequence generating the magnetic resonance signal. In this way, in particular given a spin echo signal, as far as is technically possible signal portions are also recorded that chronologically precede a maximal signal level, whereby, among other things, the signal-noise ratio is improved. For this purpose, in a further embodiment the magnetic resonance signal of a spin echo is recorded symmetrically around a spin echo time point.
In another embodiment, the time span is divided into time segments, the magnetic resonance signal of at least one of the time segments is weighted with an additional bell-shaped window function. The additional window function has an additional window width that is approximately equal to the time segment. The additional window width is placed into a region of the (at least one) time segment. The magnetic resonance signal weighted with the additional window function is subjected to an additional Fourier transformation. At least one non-dominant resonance line is made more precise or defined. In addition, in a further version the time segment is selected from an initial region of the time span. In this way, non-dominant resonance lines are made more precise in their amplitude and frequency, these lines being recorded only at the beginning of the time span due to their short decay time, and experiencing a strong suppression due to the window function.